Cochlear mechanical measurements are crucial to understanding many of clinical and psychophysical phenomena. There are many ways to 'explain' cochlear mechanics that have little to do with actual I physics of hearing when there is insufficient data to constrain interpretations. A central goal of these studies is to provide the database for interpreting observations obtained from physiological studies of the cochlear nerve or from psychophysical studies from which data are subsequent to transformations engendered by the auditory periphery. Most cochlear mechanical measurements have been made in the basal region of the cochlea while the apical region is where most human communication sounds are processed. Measurements will be made in both the apical and mid-frequency regions to provide direct observations of mechanics in this critical region using a displacement measuring laser interferometer. These measurements should settle whether cochlear amplification exists and decreases continuously From base to apex. An outcome the compressive nonlinearity in cochlear mechanics is that the response to complex signals cannot be predicted based on pure-tone responses. Proposed studies are designed to determine the representation of multi-tone and speech-like (two-formant) signal complexes in the base, middle and apical region of the cochlea. The transient response of the basilar membrane in these regions will be determined by directly driving the stapes with an ideal impulse delivered from a piezocrystal driver to avoid the contribution of the middle ear. Distortion product otoacoustic emissions and mechanical vibrations will be measured simultaneously in both normal animals and an animal model of auditory neuropathy. Crossed efferent modulation of the cochlea will be studied using acoustic stimulation of the contralateral ear in a decerebrate subject. Mechanical measurements will be compared I to auditory nerve responses in the same species and using the same signals.